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Tuesday, March 28, 2017

April's Tornadic, Dangerous Reputation

Jonathan Belles, Jon Erdman
Published: March 28,2017
While
tornadoes can be spawned any time of year the volatile mix of
climatological ingredients exist, April is a particularly dangerous
month for tornadoes in parts of the U.S.
In the period between
1991-2015, April comes in only behind May and June for average U.S.
tornado counts. That average is admittedly skewed by April 2011, in
which 758 U.S. tornadoes set a record for any month.(MORE: Tornado Central)
A
swath from the central and southern Plains east into the Tennessee
Valley is most susceptible to swarms of tornadoes in the first full
month of spring. However, the tornado threat also rises markedly in
April much farther north into the Mississippi and Lower Ohio Valleys.

Typical April tornado risk areas. Areas typically with the highest risk of tornadoes in April are contoured white.

Heading through spring, the sun is increasingly
higher in the sky, so there is more daylight available to warm the
Earth's surface.
Ahead of frontal systems swinging through the
Plains and South, temperatures will warm into the 70s, 80s, or perhaps
90s in April than in March or certainly February and January.
Coincident
with warming temperatures are increases in both the magnitude and depth
of moist air ahead of the aforementioned frontal systems. By April,
surface dew points in the 60s, or even 70s, penetrate farther north,
supplying buoyancy for severe thunderstorms to grow.(VORTEX-SE Investigates: Tornadoes in the Southeast May Be Influenced by Mountainous Terrain)
Overtopping this increasingly warm and humid pre-frontal air mass is a still-powerful jet stream.
An average April jet stream is strongest over the southern U.S., over that warm and humid air.
A
common outbreak scenario involves a pronounced southward dip in the jet
stream, or upper-level trough, that plows east into the Plains or South
with winds aloft spreading apart, forcing strong upward vertical motion
in the atmosphere.
The
jet stream also provides deep wind shear, or changing wind speed and
direction with height, supportive of rotating supercell thunderstorms.
If
wind shear is particularly strong in the first few thousand feet near
the surface, these supercells would more likely produce tornadoes.
Check our Tornado Central page for the latest on the current severe weather threat.

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